The invention relates to a process for preparing impact-modified plastics, and also to an impact-modified plastic obtainable using the process.
Impact-modified plastics have improved mechanical properties, making them particularly suitable for many applications, for example consumer articles. These particular properties are achieved through the structure of these plastics, in which elastomer domains, for example rubber domains, have been embedded in a matrix made from the thermoplastic. The presence of more than one phase in these impact-modified plastics, and therefore also their domain structure, arises due to their build-up from different polymeric components which are immiscible, or only partially miscible, with one another. Their impact-strength is a result of absorbing more energy during deformation before fracture occurs. This energy is used in forming microcavities or for inducing sliding effects within the matrix polymer chains. The presence of more than one phase is therefore a necessary precondition for achieving high impact strengths.
Other factors which apply are as follows:
1. The two chemically different polymeric components generally form a stable dispersion with a defined particle size, showing only little phase separation during processing and without any tendency to homogenize with formation of a macromolecular solution on exposure to relatively intense heat.
2. There must be some coupling present between the elastomer particles and the matrix, that is to say the phase boundaries must be capable of transferring forces.
The most effective coupling at the elastomer particle interfaces is achieved by graft copolymerization. The process here is generally to take a rubber and then to graft a copolymer onto this by polymerizing with a monomer mixture.
DE-A-2 910 168 discloses stable, flowable dispersions of rubbers in the form of discrete particles of average diameter from 100 to 3000 nm in organic liquids in which the rubber present is from 1 to 20% by weight (based on the entire dispersion) of a crosslinked diene rubber, and from 0 to 20% by weight of water in the form of a water-in-oil emulsion is present. As continuous organic phase they comprise from 99 to 66% by weight of C1-C10-alkyl acrylates or alkyl methacrylates, methyl methacrylate, ethyl acrylate or n-hexylacrylate. As an alternative, they may also comprise, as liquid phase, a mixture of from 85 to 50% by weight of styrene or x-methylstyrene and from 15 to 50% by weight of acrylonitrile, methacrylonitrile or C1-C6-alkyl acrylates or, respectively, alkyl methacrylates, such as methyl methacrylate, ethyl acrylate or n-hexyl acrylate. A further alternative proposed for the continuous organic phase is a mixture of from 85 to 50% by weight of C1-C10-alkyl acrylate or, respectively, alkyl methacrylate and from 15 to 50% by weight of acrylontrile, methacrylonitrile or styrene. The continuous organic phase may in each case comprise up to 60% by weight of an admixed liquid hydrocarbon. Aqueous emulsions of crosslinked, rubbery diene polymers are dispersed in certain organic liquids, and the result is dispersion of the diene polymers in the form of swollen particles. The water of the initial emulsion is likewise present in dispersed form in the organic liquid, as a water-in-oil emulsion. If required, the water may be removed by selectively breaking the water-in-oil emulsion and separating off the water. However, for most applications of the organic rubber dispersions there is no requirement to separate off the water, but it must be in stable dispersed form and not separate out as a distinct phase.
DE-A-4 440 676 describes a process for preparing rubber molding compositions, and also rubber-modified molding compositions prepared by the process. Here, a first stage polymerizes a mixture which comprises at least one alkyl acrylate or alkyl methacrylate, a first monomer having two or more double bonds and, if desired, one or more second monomers, to give a rubber, preferably using free radicals. A second stage dissolves, or swells, the resultant rubber in one or more third monomers to give a second mixture, if desired with addition of a solvent, and a third stage polymerizes the second mixture formed by the dissolution or swelling, to give the rubber-modified molding composition. Examples of first monomers are allyl methacrylate, butanediol diacrylate, divinylbenzene, triallyl cyanurate and dihydrodicyclopentadienyl acrylate, and the latter is preferred. The dienes used therefore comprise compounds with isolated double bond. Examples given for the second monomers are styrene, acrylonitrile, acrylic acid, methacrylic acid, and also derivatives of the two latter monomers.
DE-A-2 400 659 describes a process for preparing rubber-modified resins. Here, an alkadiene rubber, which has been grafted with monovinylidene-aromatic monomers and with alkene nitrile monomers, is dispersed into a hot melt of a base copolymer composition of monovinylidene-aromatic monomers and alkene nitrile monomers. The impact-modified plastic is therefore prepared through a physical mixing procedure. The grafting required for adequate force-transfer between polymer matrix and rubber has to be carried out in a separate step.
U.S. Pat. No. 3,957,912 describes a process for preparing an acrylonitrile-butadiene-styrene plastic. Here, an alkyl-diene rubber is first polymerized with styrene and/or acrylonitrile monomers by emulsion polymerization, to give a grafted rubber. Styrene and/or acrylonitrile are then added to this rubber, as is at least one solvent for the styrene-acrylonitrile copolymers. The rubber is taken up in the solvent, the water is separated off, and the mixture of rubber, solvent and monomer is polymerized. The transfer of the rubber into the solvent makes this process complicated to carry out.
U.S. Pat. No. 3,903,199 and U.S. Pat. No. 3,903,200 describe processes for preparing acrylonitrile-butadiene-styrene polymers. Here, particles of a first grafted rubber are dispersed in a mixture of a monovinylidene-aromatic monomer and an alkene nitrile monomer. The process consists in either first partially polymerizing the mixture and then adding particles of a second grafted rubber and polymerizing the matrix to completion or in adding the particles of the second grafted rubber directly to the mixture of the monomers and polymerizing the matrix to completion. In both cases the plastic obtained has bimodal size distribution of the rubber particles.
Due to the particular properties of impact-modified plastics, their wide application and their resultant economic importance, there is a constant requirement for new and improved plastics of this type.
It is an object of the present invention, therefore, to provide a process for preparing impact-modified plastics, and also an impact-modified plastic obtainable by this process.
We have found that this object is achieved by means of a process for preparing an impact-modified plastic, where
in a first step, particles of a crosslinked rubber are produced from a first monomer mixture which has at least 50% by weight of conjugated diene compounds,
the rubber particles are added to a second monomer mixture which has at least 85% by weight, preferably at least 90% by weight, in particular at least 98% by weight, of styrene, and
the monomers of the second monomer mixture are polymerized.
The first step of the process produces a rubber in particle form, onto which the monomers of the second monomer mixture are then grafted. This gives effective phase coupling at the interfaces between the elastomer particles and the surrounding polymer matrix. No separate step is required to polymerize-on the graft shell. The high proportion of conjugated diene compounds in the rubber gives better resistance to mechanical effects, and this is also true at relatively low temperatures. There is no requirement for the dispersion of the rubber to be stable. In many cases the swelling of the rubber particles in the second monomer mixture causes gel formation. During polymerization of the second monomer mixture phase separation occurs, making the product flowable. There is no requirement for grafting of the rubber prior to the polymerization of the second monomer mixture in order to obtain the desired mechanical properties.
Examples of conjugated diene compounds suitable for preparing the rubber are butadiene, isoprene, 2-chloro-1,3-butadiene, 1-chloro-1,3-butadiene, and also other substituted butadienes and isoprenes. The rubber dispersion may be prepared by emulsion, miniemulsion or microsuspension methods, for example. The rubber may be added as an aqueous dispersion, but it is also possible for the water to be removed and the rubber particles to have a water content below 5% by weight when added to the second monomer mixture. The rubber particles may also first be coagulated and, after separating off some of the water, have a water content of from 5 to 60% by weight when added to the second monomer mixture. The separation of the water from the rubber particles may take place by pressure filtration, centrifuging or drawing off the water at reduced pressure, for example.
There are various ways of adding the rubber dispersion to the second monomer mixture. The dispersion may be directly added to the monomer mixture, but it is also possible for one of the monomers to form an initial charge and then for the rubber dispersion to be added, followed by the other monomers of the monomer mixture. The latter method is useful when, for example, the further monomer of the second monomer mixture is more volatile than water and the water is to be separated off prior to starting polymerization of the second monomer mixture. In this case, a less volatile monomer of the second monomer mixture, for example styrene, forms an initial charge. The aqueous rubber dispersion is then added and the water separated off, for example by distillation. The more volatile monomers, such as acrylonitrile, may then be added so that the second monomer mixture can then be polymerized in the third step.
The water deriving from the rubber dispersion may be removed from the reaction mixture in various ways. It may be distilled off, either prior to the polymerization or else during the polymerization of the second monomer mixture. In the latter case, the evaporating water can serve for evaporative cooling. It is also possible for the water to be removed by decanting after the rubber dispersion has been broken.
The second monomer mixture may also comprise only styrene, that is to say may also be free from admixtures. This improves the color of the product. The process is moreover simpler to carry out, since in comparison with acrylonitrile, styrene is considerably less water-soluble and volatile in water vapor. Smaller amounts of monomers are therefore lost during removal of the water.
In one particular embodiment of the process, the second monomer mixture may also comprise at least one further polymer which is preferably compatible or partially compatible with the polymer obtained from the second monomer mixture. For the purposes of the present invention, compatibility implies that no phase separation occurs between the at least one further polymer and the polymer obtained from the second monomer mixture. One way of preparing the further polymer is to polymerize the second monomer mixture to some extent and then add the rubber dispersion to the partially polymerized monomer mixture, and then to complete the polymerization of the second monomer mixture.
The further polymer preferably has a glass transition temperature Tg of 0xc2x0 C., preferably  greater than 20xc2x0 C., in particular  greater than 50xc2x0 C.
The rubber particles may have a hard core made from a copolymer which preferably has a glass transition temperature  greater than 0xc2x0 C., particularly preferably  greater than 10xc2x0 C., in particular  greater than 20xc2x0 C. This hard core may be composed of polystyrene, for example.
The hard core preferably has a refractive index above 1.53, preferably above 1.56, in particular above 1.57. Impact-modified plastics which comprise small rubber particles are mostly opaque. They are therefore very difficult to color. The hard core enables the refractive index of the rubber particles to be brought close to that of the surrounding polymer matrix, reducing light scattering. This equalizing process is particularly effective with a hard core which comprises styrene or comprises a styrene derivative. These polymers have particularly high refractive index.
The size of the rubber particles in the polymer matrix is preferably below 10 xcexcm, with preference below 5 xcexcm, in particular below 4 xcexcm.
The swelling index of the rubber particles is preferably from 2 to 200, with preference from 3 to 100, in particular from 5 to 90. The swelling index is determined as follows:
A film is cast using the dispersion of the grafted crosslinked rubber particles, and the water removed by evaporation at 23xc2x0 C. The film is then dried at reduced pressure and 50xc2x0 C. About 0.5 g of the film is swollen for 24 hours in a solvent, such as tetrahydrofuran or dimethylformamide. The polymer gel is then separated by centrifuging from the solvent not bound into the gel. The gel is weighed, then dried and weighed again. The swelling index (SI) is calculated from the following equation:   SI  =            Weight      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      swollen      ⁢              xe2x80x83            ⁢      polymer      ⁢              xe2x80x83            ⁢      gel              Weight      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      dried      ⁢              xe2x80x83            ⁢      polymer      ⁢              xe2x80x83            ⁢      gel      
In one preferred embodiment, the rubber particles are in the form of a dispersion in water when added to the second monomer mixture, and, if desired, the mixture made from the rubber particles in water and from the second monomer mixture is homogenized, in particular by high-speed stirring.
After the rubber and the monomers have been mixed, the monomers may be polymerized immediately, or the rubber may first be swollen with the monomers and then the polymerization be carried out.
Up to 80% by weight, preferably up to 60% by weight, in particular up to 20% by weight, of a solvent may have been added to the second monomer mixture. Examples of suitable solvents are toluene, ethylbenzene, dimethylformamide, acetone, etc.
The polymerization may be carried out in bulk or in solution. The polymerization of the second monomer mixture may also advantageously be carried out in suspension. The resultant reaction mixture is flowable and easier to process. It is also possible for the reaction in solution or bulk to be converted to a reaction in suspension during the polymerization.
The impact-modified plastic obtained using the novel process has high mechanical strength. It may also be present in a mixture with at least one additional synthetic polymer. Suitable additional synthetic polymers are polycarbonates, polyesters, polyamides, polyalkyl methacrylates, where for the purposes of the present invention these are either homo- or else copolymers, and also high-temperature-resistant polysulfones and high-temperature-resistant poly(ether) sulfones. Other polymers are polypropylene, polyethylene, polytetrafluoroethylene (PTFE) and polystyrene-acrylonitrile. Preference is given to polyphenylene ether (PPE), syndiotactic polystyrene, styrene-diphenylethylene copolymers, and also copolymers having a styrene content above 85% by weight.
Besides the components described, the rubber-modified molding compositions may also comprise the usual amounts of additives, such as lubricants, mold-release agents, pigments, dyes, flame retardants, antioxidants, stabilizers to prevent damage by reaction of light, fibrous or pulverulent fillers, fibrous or pulverulent reinforcing materials, or antistats.